The invention relates to methods of producing ions. The ions produced may be used in the field of mass spectrometry.
In biological mass spectrometry (MS), two ionization techniques are predominantly employed for the analysis of analytes which are larger biomolecules, for example polypeptides. These are nanoelectrospray ionization (nanoESI) and matrix-assisted laser desorption/ionization (MALDI). In MALDI a laser is used to ablate a matrix/analyte material to release ions into the gas phase. These ions are then passed into a mass analyzer/spectrometer. Both techniques are considered to be ‘soft’, allowing the desorption and ionization of intact molecular analyte species and thus their successful mass spectrometric analysis. One of the main differences between these two ionization techniques lies in their ability to produce multiply charged ions. MALDI typically generates singly charged ions when used with peptide analytes while nanoESI easily provides multiply charged ions, even for peptides as low as 1,000 Da in mass. The production of highly charged ions is desirable as this allows the use of mass analyzers such as ion traps (including orbitraps) and hybrid quadrupole instruments, which typically offer only a limited m/z range (<2,000-4,000). It also enables more informative fragmentation spectra using techniques such as collision-induced dissociation (CID) and electron capture/transfer dissociation (ECD/ETD) in combination with tandem MS (MS/MS). The MALDI technique can be preferable as it has higher tolerance to contaminants and additives, ease-of-operation, potential for high-speed and automated sample preparation and analysis as well as MS imaging capabilities. Thus, MALDI is an ionization technique that can cover bioanalytical areas where ESI is less suitable. A MALDI technique which can produce multiply charged ions is therefore desirable.
Previous MALDI methods which have applied a laser to a liquid matrix/analyte system are described in ‘Liquid ultraviolet matrix-assisted laser desorption/ionization-mass spectrometery for automated proteomic analysis, R. Cramer and S. Corless Proteomics 2005, 5, 360-370’ and ‘Employing target modifications for the investigation of liquid infrared matrix-assisted laser desorption/ionization mass spectrometery, R. Cramer and A. L. Burlingame, Rapid Commun. Mass Spectrom. 14, 53-60 (2000)’. Neither of these methods produced desirable amounts of multiply charged ions.